The flux method is one of liquid phase methods. For gallium nitride, the use of sodium metal as a flux can lower the temperature and pressure, which are required for the crystal growth of gallium nitride, to about 800° C. and several MPa. Specifically, nitrogen gas is dissolved in a mixed melt of sodium metal and gallium metal to bring gallium nitride into a supersaturated state, so that gallium nitride grows as crystals. In such liquid phase method, dislocation hardly occurs compared with vapor phase methods, thereby allowing to obtain high-quality gallium nitride having a low dislocation density (Patent document 1 (JP 2009-012986 A)).
The present applicant disclosed in Patent document 2 (JP 5607548 B2) a crystal growth apparatus suitable for the mass production of Group 13 element nitrides using the flux method. In this apparatus, a pressure-resistant vessel connected to a nitrogen gas pipe is provided with a heating space in its inside. A inner vessel is placed over a turntable arranged inside the heating space. Further, the inner vessel is provided with a nitrogen gas inlet port that is not connected to the nitrogen gas pipe installed to the pressure-resistant vessel. The inner vessel having such a configuration is configured to rotate during the growth of crystals to achieve an improvement in crystal quality.